ok, in your stylesheet file, you scroll down until you get to this section


HTML Code:
/*sideboxes*/
.columnLeft {}

h3.leftBoxHeading, h3.leftBoxHeading a,
h3.rightBoxHeading, h3.rightBoxHeading a {
	font-size: 1em;
	color: #ffffff;
	}
#manufacturersHeading, #currenciesHeading, #musicgenresHeading, #recordcompaniesHeading, #searchHeading, #search-headerHeading {
	font-size: 0.9em;
	color: #ffffff;
}

.leftBoxHeading, .centerBoxHeading, .rightBoxHeading {
	margin: 0em;
	background-color: black;
	
	}

.leftBoxContainer, .rightBoxContainer {
	margin: 0em;
	border: 1px solid #5ABEE7;
	border-bottom: 5px solid #5ABEE7;
	margin-top: 1.5em;
	}

.sideBoxContent {
        border-top: 1px solid #5ABEE7;
	background-color: black;
        background-image: url(../images/cyber.gif);
	padding: 0.4em;
	}
then, under .sideBoxContent, you add

HTML Code:
background-image: url(../images/YOURBACKGROUNDIMAGE.gif);
the end